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Night vision
Night vision is the ability to see in a dark environment. Most instances, whether biological or technological, use a combination of two approaches: enhanced spectral range, and enhanced intensity range. Enhanced spectral range Enhanced spectral range techniques make the viewer sensitive to types of light that would be invisible to a human observer. Human vision is confined to a small portion of the electromagnetic spectrum called visible light. Enhanced spectral range allows the viewer to take advantage of non-visible sources of electromagnetic radiation (such as near-infrared or UV radiation). Enhanced intensity range Enhanced intensity range is simply the ability to see with very small quantities of light. Although the human visual system can, in theory, detect single photons under ideal conditions, the neurological noise filters limit sensitivity to a few tens of photons, even in ideal conditions http://math.ucr.edu/home/baez/physics/Quantum/see_a_photon.html. Some animals have evolved better night vision through the use of a larger optical aperture, improved retina composition that can detect weaker light over a larger spectral range, more photoefficient optics in the eye, and improved neurological filtering which is more tolerant of noise. Enhanced intensity range is achieved via technological means through the use of an image intensifier, gain multiplication CCD, or other very low-noise and high-sensitivity array of photodetectors. Biological night vision In biological night vision, molecules of rhodopsin in the rods of the eye undergo a change in shape as light is absorbed by them. The peak rhodopsin build-up time for optimal night vision in humans is 30 minutes. Rhodopsin in the human rods is insensitive to the longer red wavelengths of light, so many people use red light to preserve night vision as it will not deplete the eye's rhodopsin stores in the rods and instead is viewed by the cones. Some animals, such as cats, dogs, and deer, have a structure called the tapetum in the back of the eye that reflects light for even better night vision than humans, in which only 10% of the light that enters the eye falls on photosensitive parts of the retina. Thermal vision Far infrared, or thermal, sensing is generally not considered night vision because it is constructed with mechanisms substantially different from the methods used to sense visible light. Some organisms have the ability to sense far infrared energy which we perceive as heat. This is prevalent in some snakes such as pit vipers and boas. However, this is not actual "vision", but more of a system of thermosensitive pits in the face that can detect the amount of heat and the distance to the heat source. There is still some debate as to what degree this information is perceived as "feeling" heat, and to what degree it is processed as an image by the snake's brain. Since these sense structures lack imaging optics for focus, the spatial resolution of such sensing is necessarily very poor. See also * Scotopic vision * Night vision goggles External links Category:Infrared imaging da:Nattesyn es:Visión nocturna fr:Nyctalopie he:ראיית לילה id:Penglihatan malam it:Visione notturna nl:Nachtzichtapparatuur ja:ナイトビジョン no:Nattsyn pt:Visão noturna